We know it's a genuine movement, because it's developed it's own fashionable nonsense.
They speak of a four-way distinction:
The terms are not exactly clear, and not consistently used, but it seems to come down to this:
It's an incredibly unhelpful distinction. Because 99.9% of atheists are 'agnostic' in this sense, and so are 99.9% of believers.
This 'gnostic'/'agnostic' distinction doesn't describe the belief itself, but an attitude towards hypothetical future evidence.
This makes no more sense than dividing vegetarians and meat-eaters according to whether they think they might change their eating habits if the nature of meat were to change.